1. Field of the Invention
The present invention relates generally to gain control and calibration of transmitters.
2. Background Art
3G/4G wireless protocols require power control on the handset (also known as user equipment (UE) in 3G/4G) to increase the overall throughput of the wireless network. This requirement is divided into two cases. In the first case, the handset should be capable of setting its output power to a specific value. The power control tolerance in this case is not stringent (e.g., 8-9 dB) and can be met by good design practice. In the second case, called inner loop power control, the handset should be able to adjust its output power in accordance with a transmit power command (TPC) received from a base station. In 3G, the inner loop power step is defined as the relative power difference between the mean power of the original (reference) timeslot and the mean power of the target timeslot in each carrier, not including the transient duration. The transient duration is defined as the duration from 25 μsec before the slot boundary to 25 μsec after the slot boundary.
3G/4G protocols allow a maximum error of ±0.5 dB in gain over all conditions (e.g., temperature, process, VSWR, supply voltage, etc.) when a 1 dB power level change is made. This gain step accuracy should be met over the entire transmitter power control range of 80 dB or higher.
Another critical requirement for transmitter design for 3G/4G is power consumption, which governs the amount of time that the handset can be used without re-charging. DG09 current is a common metric used in the industry to calculate current consumption based on a 3G user profile. DG09 places the most emphasis on the power levels at which the handset is operated for largest percentage of time. Accordingly, in order to reduce DG09 current consumption, a common practice in today's transmitter design is to switch on/off various analog blocks of the transmitter chain for different output power levels.
The majority of conventional gain control and correction schemes for 3G/4G transmitter chains are based solely on open loop control, where power control relies on accurate characterization and/or factory calibration of analog gain steps within the transmitter chain in order to meet the aforementioned gain step accuracy of ±0.5 dB for a 1 dB power level change. In particular, these conventional designs rely on look up tables for storing the characterization/calibration results and applying them during normal operation. While some conventional schemes also use closed loop power control (using the coupler and detector of the power amplifier), they generally limit closed loop power control to operation at higher output power levels (e.g., above 5-8 dBm) due to the low linear dynamic range of the detector. In addition, certain conventional solutions involve complicated designs in order to restrict potential analog gain variations (due to temperature, frequency, bias, etc.) to meet performance requirements such as noise and linearity, for example.
Hence, existing gain control and calibration approaches significantly complicate the design of transmitter circuitry, make maintaining performance over large volume production more difficult, and require additional verification during production ATE testing.
Accordingly, there is a need for improved gain control and calibration techniques for 3G/4G transmitters.
The present invention will be described with reference to the accompanying drawings. Generally, the drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.